At present, plastic containers (bottles, etc.) made mainly of polyesters such as polyethylene terephthalate (PET) have been extensively used as containers for teas, fruit juices, carbonated beverages, etc. Among these plastic containers, small-size plastic bottles have increased in proportion year by year. In general, as the size of a bottle is reduced, a surface area thereof per unit volume tends to increase. Therefore, a gustoish period of contents in the small-size bottles tends to be shortened. In recent years, beer susceptible to influences of oxygen and light as well as hot tea which are filled in a plastic bottle have been put on the market. Thus, with the recent tendency that the plastic containers are used in more extensive applications, the plastic containers have been required to be further improved in gas-barrier property.
To meet the above requirement for imparting a good gas-barrier property to plastic bottles, there have been developed multilayer bottles produced from thermoplastic polyester resin and gas-barrier resin, blend bottles, barrier-coated bottles produced by forming a carbon coat, a deposited coat or a barrier resin coat onto a single layer bottle made of a thermoplastic polyester resin, etc.
The multilayer bottles, for example, those bottles produced by subjecting a three- or five-layer preform (parison) obtained by injecting a thermoplastic polyester resin such as PET for forming innermost and outermost layers and a thermoplastic gas-barrier resin such as poly-m-xylyleneadipamide (polyamide MXD6) into a mold cavity, to biaxial stretch blow-molding, have been put into practice.
Further, resins having an oxygen-capturing function for capturing oxygen within the container while preventing penetration of oxygen from an outside of the container have been developed and applied to multilayer bottles. The oxygen-capturing bottles are suitably in the form of a multilayer bottle including a gas-barrier layer made of polyamide MXD6 in which a transition metal-based catalyst is blended, from the viewpoints of oxygen-absorbing velocity, transparency, strength, moldability, etc.
The above multilayer bottles have been used as containers for beer, tea, carbonated beverages, etc., because of their good gas-barrier property. When the multilayer bottles are used in these applications, contents filled therein can maintain a good quality with an improved shelf life. On the other hand, the multilayer bottles tend to undergo delamination between different resin layers, for example, between the innermost or outermost layer and the intermediate gas-barrier layer, resulting in significant damage to their commercial value. In particular, when the multilayer bottles are formed into an irregular shape for the purposes of imparting a good design property thereto or enhancing a strength thereof, there tends to arise such a problem that irregular portions of the bottles suffer from delamination.
To solve the above problems, there has been proposed such a method in which when a resin for forming the innermost and outermost layers is finally injected into a mold cavity, a given amount of the resin is flowed in a reverse direction toward the side of the intermediate gas-barrier layer using a reverse-flow controller to produce a preform containing a coarse mixed resin introduced between the layers, thereby improving a delamination resistance of the resultant multilayer bottle (refer to Patent Document 1). However, in this method, it is required to use the special apparatus. Also, there has been proposed the method for producing a multilayer bottle by a stretch blow molding method in which a preform once blow-molded is contracted under heating and then subjected again to blow molding under a high pressure (refer to Patent Document 2). However, in this method, there tend to occur problems such as defective shape of the resultant molded product, complicated time-consuming procedure and insufficient delamination resistance.
Patent Document 1: JP 2000-254963A
Patent Document 2: JP 2001-206336A